Infrared gas sensors use an infrared energy source to detect the presence of an analyte in an environment being tested. In general, the analyte absorbs infrared energy of a certain wavelength and this absorption can be quantified to determine the concentration of the analyte in the test environment. Several embodiments, of infrared fluid sensors or analyzers are discussed, for example, in U.S. Pat. Nos. 4,355,234, 4,598,201 and 4,902,896, assigned to the assignee of the present invention, the disclosures of which are incorporated herein by reference.
An example of a commercially available infrared gas sensor is the ULTIMA® XIR Gas Monitor, available from Mine Safety Appliances Company of Pittsburgh, Pa. The operation of that sensor is discussed in detail in the ULTIMA X Series Gas Monitors Instruction Manual available from Mine Safety Appliances Company, Instrument Division, the disclosure of which is incorporated herein by reference.
In general, the ULTIMA XIR Gas Monitor uses an electronically modulated source of infrared energy and two detectors that convert the infrared energy into electrical signals. The source of infrared energy emits light energy over a broad spectrum of wavelengths, including visible light. The light passes through two different wavelength filters. Each detector is sensitive to a different range of wavelengths in the infrared portion of the light spectrum. The source emission is directed through a window in the main enclosure into an open volume. A mirror at the end of this volume, protected by a second window, directs the energy back through the window in the main enclosure and onto the detectors. During operation of the sensor, the open volume is surrounded by a cap through which environmental gas can pass, but which protects the volume from exposure to wind and other environmental elements that might cause erroneous or erratic readings. The presence of an analyte gas in the open volume reduces the intensity of the source emission measured by the analytical detector but not the intensity of the source emission measured by the reference detector. In that regard, the reference detector measures infrared intensity at a wavelength outside of the analyte absorption range. A microprocessor monitors the ratio of the two signals and, in the case of a combustible gas, for example, correlates the ratio into a % Lower Explosion Limit (LEL) combustible reading.
Typically, infrared gas sensors are initially calibrated by the manufacturer. However, periodic recalibration of the sensor is often desirable. Under current practice, calibration techniques for infrared sensors require the use of an intelligent external calibration device. Such calibration devices typically communicate with a main monitor unit to which the infrared sensor is attached at the sensing location. The main monitor unit typically includes a user feedback display and appropriate communications circuitry (to communicate with a control center remote from the sensor as well as with an external calibration device) enclosed within a housing that is specifically designed for use in a hazardous location (for example, the housing and display unit window can form an explosion-proof enclosure in the case of a monitor for combustible hydrocarbons). For example, an ULTIMA X Calibrator or an ULTIMA X Controller is available for use with the ULTIMA XIR Gas Monitor. Those units are hand-held, battery-powered units, that allow one person, non-intrusive calibration of the ULTIMA XIR Gas Monitor without opening the monitor or sensor housings. Use of the ULTIMA X Calibrator and Controller is described in the ULTIMA/ULTIMA X Series Controller and Calibrator Instruction Manual, available from Mine Safety Appliances Company, Instrument Division, the disclosure of which is incorporated herein by reference.
During calibration of a gas sensor, one or more sample or calibration gases having a known concentration of analyte gas (or a simulant gas) are preferably applied directly to the sensor. Typically a “zero” gas, having no analyte therein, is applied to the sensor during calibration. A “span” gas, having a known, non-zero analyte (or simulant) concentration, can also be applied during calibration. Often, the concentration of analyte (or simulant) in the span gas is approximately at the midrange of the overall concentration range of the sensor. In many cases, it is necessary to perform only a zero gas procedure in periodically calibrating an infrared gas sensor, as restoring the sensor's zero performance is typically sufficient to restore the sensor's span performance.
An intelligent calibrator such as the ULTIMA X Calibrator or the ULTIMA X Controller can communicate with the gas monitor/sensor (using, for example, infrared energy or radio frequency energy) to place the sensor in a calibration mode. Various inputs are provided on the calibrator to communicate any number of functions to the gas monitor to initiate, for example, a zero procedure, zero and span procedures, a network address change, a span value change etc. In general, such intelligent calibrators operate very well. However, relatively complicated electronics in both the calibrator and monitor are often required, resulting in substantial expense and operational complexity. Moreover, in the case of infrared sensors for point detection of combustible gas(es) the calibrator must be housed in an explosion-proof housing or be intrinsically safe, adding further expense.
It is thus desirable to develop alternative communication/calibration devices, systems and methods that are relatively inexpensive and easy to operate.